A traditional upper receiver 10 for meeting military specifications (MIL SPEC) for the AR-15 firearm is illustrated in FIGS. 1-3. Referring to FIG. 1, the upper receiver 10 includes a rear end 10a and a front end 10b. The rear end 10a of the upper receiver 10 includes a charging handle slot 12 defined between a top wall 14 and a sidewall 16 of the upper receiver 10. The top wall 14 includes a flat (i.e., planar), downwardly-facing surface 18 and the sidewall 16 of the upper receiver 10 defines a generally curved, rear-facing end wall 20 on each side of the slot 12. The generally curved end walls 20 have a relatively vertical center portion and angled top and bottom portions. The shape of the slot 12 follows relatively closely the shape of a charging handle for the firearm such that the forward-facing surface of the charging handle fits against the rear-facing end wall 20. A chamber 22 inside the upper receiver 10 is traditionally formed with a milling operation from the front end 10b of the upper receiver 10 and the slot 12 is traditionally formed with a broaching operation from the rear end 10a because the area of the slot 12 is not accessible with a traditional mill from the front end 10b. 
With reference to FIGS. 2 and 3, the front end 10b of the upper receiver 10 includes a gas tube placement hole or gas tube port 30 through which the gas tube 32 enters the upper receiver 10. The gas tube port 30 is surrounded by four symmetrical lobes 34 which communicate with the gas tube port 30. The lobes 34 accommodate misalignment of the gas tube 32 with respect to the central axis of the gas tube port 30. The upper receiver 10 includes a gas accumulation space 36 immediately rearward of the gas tube port 30, where gases expand as they enter the upper receiver 10. The gas accumulation space 36 communicates with the chamber 22.